The present invention relates to a bilateral optical transmission system and an optical transceiver for pulse information which are used in an optical access system for optical subscriber communication or optical CATV employing a high density wavelength-multiplexing system.
One possible scheme that has been proposed to economically implement an optical access system is a single cable transmission system which transmits both up- and down-link optical signals over one optical fiber as disclosed in, for example, (P. J. Duthie, M. J. Wale, I. Bennion, and J. Hankey, "Bidirectional Fibre-Optic Link Using Reflective Modulation," Electronics Letters, vol. 22, no. 10, pp. 517-518, 1986. In FIG. 15 there is schematically shown a configuration of a single cable bilateral transmission system. Assume that the transmission rates from a central office to a subscriber and from the latter to the former are 560 and 34 Mb/s, respectively.
A 560 Mb/s optical signal is sent out from an optical transmitter 501 of the central office to a fiber optic transmission line 503 via a directional coupler 502. At the subscriber side a portion of the optical signal transmitted thereto is provided via an optical coupler 504 to an optical receiver 505, where it is demodulated. The remainder of the transmitted optical signal is fed to an optical modulator 506, and the 560 Mb/s down-link optical signal is envelope-modulated using information of the transmission rate 34 Mb/s, thereafter being provided via the directional coupler 504 onto the transmission line 503. At the central office side, the up-link optical signal is separated by the directional coupler 502 from the transmitted signal and is received by an optical receiver 507. The optical receiver 507 extracts only the 34 Mb/s signal by an integration circuit. This system is predicated on a condition that the transmission rate of the down-link signal is higher than that of the up-link signal.
Turning next to FIG. 16, a description will be given of an example which employs a wavelength multiplexing system in an access network from the central office to subscribers. Let it be assumed that to subscribers 301 to 304 are assigned wavelengths .lambda..sub.1 to .lambda..sub.4 (the number of subscribers in this example is four), respectively. Optical signals of wavelengths .lambda..sub.1 to .lambda..sub.4 from optical transmitters 311 to 314 of the central office are multiplexed by an optical coupler-splitter 315 and then provided onto one fiber 316. The optical signals are split, by an optical coupler-splitter 318 in a node 317 placed near the subscribers, into signals each corresponding to one subscriber, thereafter being received by optical receivers 321 to 324 of the subscribers via optical fibers 331 to 334. Such a network is commonly called a passive double star (PDS) network. As regards a message from the subscriber, for example, 301 to the central office, an optical signal of the wavelength .lambda..sub.1 sent out from an optical transmitter 325 is transmitted over an optical fiber 341 to the node 317, where it is wavelength-multiplexed by an optical coupler-splitter 319 with optical signals from other subscribers transmitted over optical fibers 342 to 344, and the thus wavelength-multiplexed optical signals are transmitted to the central office over one fiber 329. In the central office, the multiplexed optical signals are split by an optical coupler-splitter 330 into signals of the respective wavelengths, which are fed to individual optical receivers 351 to 354.
In the optical access system employing the wavelength multiplexing scheme, the wavelengths are usually spaced 1 to 2 nm apart. The oscillation wavelength of a semiconductor laser as a light source undergoes a temperature change of 0.1 nm/.degree.C. even if it is a distributed feedback laser. If the temperature of the optical transmitter placed in the subscriber's home varies 20.degree. C., the oscillation wavelength will change by a value of 2 nm. When the wavelength of the optical transmitter 325 of the subscriber 301, initially set at a wavelength .lambda..sub.1 as shown in FIG. 17(a), changes to a wavelength .lambda..sub.1 ' due to a change in the ambient temperature of the optical transmitter 325, a crosstalk to adjacent channels will occur even if the wavelength characteristics of the optical coupler-splitter 319 placed in the node 317 of the network and the optical coupler-splitter 330 of the central office do not vary as shown in FIGS. 17(b) and (c). To avoid this, it is necessary to stabilize the wavelength of the optical transmitter placed in the subscriber's station. In many cases, a Peltier element is used for temperature control of the light source in a quest to stabilize its wavelength.
The optical coupler-splitter of the node is mounted on a conduit or mast, and it is considered that freedom from maintenance is a precondition for the design of an economical system. Even if the wavelength of the optical transmitter 325 of the subscriber 301 is stabilized at a wavelength .lambda..sub.1 as depicted in FIG. 18(a), the wavelength characteristic of the optical coupler-splitter 319 in the node 317 may sometimes shift by a value .alpha..lambda. due to adverse environmental conditions as shown in FIG. 18(b). Even if the optical coupler-splitter is formed of quartz glass, its wavelength characteristic undergoes a temperature change of 0.01 nm/.degree.C. owing to the temperature dependency of the refractive index of quartz glass. A 100.degree. C. temperature change (for example, an operating temperature of -40 to 65.degree. C. is required outdoors) will cause a wavelength change of 1 nm. That is, .DELTA..lambda.=1 nm. This influence is serious in the high density wavelength multiplexing system. Provided that the wavelength characteristic of the optical coupler-splitter at the central office side is such as shown in FIG. 18(c), the optical signal (a) of the wavelength .lambda..sub.1 from the optical transmitter 325 of the subscriber 301 is intercepted owing to the characteristic (b), and hence it does not reach the central office. Even if the optical signal is allowed to pass through the optical coupler-splitter 319 of the node 317 by changing a wavelength of the light source of the optical transmitter 325 to a value .lambda..sub.1 +.DELTA..lambda., the optical signal is inhibited from the passage through the optical coupler-splitter 330 at the central office side, and hence it does not reach the optical receiver 351 of the central office.
No proposals have been made so far on a method for controlling output -wavelength from a light source in an optical network in which the wavelength characteristics of optical coupler-splitters and optical filters vary due to an ambient temperature change and on a wavelength multiplexing optical access system utilizing the method.
The traffic speed or transmission rate from the subscriber to the central office differs greatly for each subscriber. It is difficult to deal with this problem by the prior art as the transmission rate of the up-link signal approaches the down-link transmission rate. For example, when the subscriber is a TV station which distributes TV signals, the transmission rate of the down-link signal is lower than the transmission rate of the up-link signal, with which the prior art cannot cope. The optical access system needs to meet every subscriber's requirement about the transmission rate.
In ordinary optical access systems using the passive double star (PDS) scheme, optical coupler-splitters are provided in nodes at the central office side and at midpositions in the transmission lines for the up-link optical signal from the subscriber to the central office as well as for the down-link optical signal from the central office to the subscriber. Even if such optical coupler-splitters exhibit the same wavelength characteristic when placed in the same environment, their wavelength characteristics change when they are disposed in different environments. In such a situation, the wavelength multiplexing communication may sometimes be impossible; this problem becomes more severe in higher density wavelength multiplexing communications. The control function, which the optical transmitter of the subscriber is required to possess so as to overcome the problem, is not only control for wavelength stabilization of a light source but also wavelength control while monitoring variations in the wavelength characteristic of the network. This puts a heavy burden on the subscriber's terminal and hence inevitably raises its cost.